Circuit arrangement for reducing the dynamic range of audio signals

ABSTRACT

Circuit arrangement having a first transformation device ( 3 ) to which an incoming audio signal ( 2 ) is supplied and which transforms this audio signal ( 2 ) from the time domain to the frequency domain resulting in an input spectrum ( 4 ), a spectral processing device ( 5 ), which is connected downstream from the first transformation device ( 3 ), to receive the input spectrum ( 4 ) and use it to produce an output spectrum ( 6 ) such that the output spectrum ( 6 ) has a narrower dynamic range than the input spectrum ( 4 ), and a transformation device ( 16 ), which is connected downstream from the spectral processing device ( 5 ), is supplied with the output spectrum ( 6 ) and transforms this output spectrum ( 6 ) from the frequency domain to the time domain, resulting in an output audio signal ( 17 ).

The invention relates to a circuit arrangement for reducing the dynamicrange of audio signals.

One problem that occurs frequently in audio systems is that the systemsare driven too strongly, and this can lead to undesirable distortionand, in some circumstances, even to damage to the system. Furthermore,it may also be desirable to limit the level of transmission systems inorder, for example, to avoid any adverse effect on the hearing oflisteners. On the other hand, in an environment filled with noise, itmay be desirable to emphasize relatively quiet passages to such anextent above the background noise that they can be perceived by thelistener.

Irrespective of whether the aim is to emphasize low levels (for exampleby means of so-called compounders) or to limit high levels (for exampleby means of so-called limiters), the outcome in both cases is that thedynamic range of the audio signal is reduced, that is to say thedifference between the minimum and maximum levels is reduced. So-calleddynamic range compression is advantageous in particular in motorvehicles since in this situation, on the one hand, the background noiselevel is very high, which can be improved by emphasizing the low levels,and on the other hand the power of the audio system is limited owing tothe low supply voltage in motor vehicles, and this can easily lead todistortion at high levels, which can be counteracted by limiting thelevels.

One problem that occurs in normal limiters or compounders is that theynecessarily always adapt themselves to the highest-energy componentwithin the audio signal, such as bass drums or snare drums, and in theprocess cause the “volume pumping” which has been known for a long time.

In order to counteract this, U.S. Pat. No. 5,255,324 proposes, forexample, that the audio signal be subjected to both narrowband andbroadband evaluation, with the process being based initially on thenarrowband audio signal and, if this does not result in a satisfactoryimprovement, broadband evaluation being carried out. U.S. Pat. No.6,005,953 discloses the provision of a number of frequency bands whichare evaluated individually, with the gain of the overall audio signalthen being raised or lowered. Although these measures in themselvesresult in an improvement, it is, however, not sufficient in many cases.

The object of the invention is thus to specify a circuit arrangement forreducing the dynamic range of audio signals, which is further improvedin this regard.

The object is achieved by a circuit arrangement according to patentclaim 1. Refinements and developments of the idea of the invention arethe subject matter of dependent claims.

In detail, the object is achieved by a circuit arrangement having afirst transformation device to which an incoming audio signal issupplied and which transforms this audio signal from the time domain tothe frequency domain resulting in an input spectrum, a spectralprocessing device, which is connected downstream from the firsttransformation device, to receive the input spectrum and use it toproduce an output spectrum such that the output spectrum has a narrowerdynamic range than the input spectrum, and a transformation device,which is connected downstream from the spectral processing device, issupplied with the output spectrum and transforms this output spectrumfrom the frequency domain to the time domain, resulting in an outputaudio signal.

In this case, the transformation device preferably operates using thefast Fourier transformation (FFT) method, and the second transformationdevice operates using the inverse fast Fourier transformation (IFFT)method. Fast Fourier transformation is characterized by a good ratio ofcomplexity to usefulness. For certain applications, a Fourier timetransformation (FTT) and its inverse function may also be used, forexample, instead of fast Fourier transformation.

In order to provide a limiter function, the spectral processing devicemay attenuate the amplitude in those areas of the spectrum in which theamplitude and/or the spectral range satisfy/satisfies one or moreconditions.

One such condition may, for example, be that the amplitude is greaterthan the first limit value in this spectral range. This results inabsolute peaks, that is to say spectral lines of a specific amplitude,being attenuated, for example, such that the relevant spectral lineremains below a specific amplitude (attenuation of absolute peaks).

As an alternative or as an additional condition, it is possible toprovide for a signal at a specific amplitude and in a specific spectralrange not to be audible. The amplitude of the overall signal can thenadvantageously be reduced by removing from the overall signal spectralcomponents which are in any case not audible. Time and/or spectralmasking in specific spectral ranges may be used, for example, aspsychoacoustical effects in this context. In this case, it isadvantageous to subdivide the spectrum corresponding to the frequencygroups of human hearing.

Alternatively or additionally, one condition may also be that therespective amplitude is a maximum or one of the maxima. In this way, thegreatest amplitudes are attenuated in a preferred manner (attenuation ofrelative peaks).

Furthermore, additionally or alternatively, one condition may be givenby the position of the respective spectral range in the overallspectrum. This applies in particular to very low or very high tonessince these may either be perceived only very weakly by the listener,may not be transmitted (completely) by the acoustic system, or may becorrupted by the room in which they are being listened to.

For dynamic compression, especially for low levels, it is possible toprovide for relatively small amplitudes which are greater than a secondlimit value which corresponds to a specific background noise level andare less than a third limit value which is higher than the second limitvalue to be emphasized.

The input spectrum and possibly also the output spectrum are preferablycomplex, so that the phases are also taken into account in theevaluation process.

Finally, in one development of the invention, the first transformationdevice may be preceded by an advanced initiation device which suppliesthe incoming audio signal with a delay to the first transformationdevice, with both transformation devices as well as the processingdevice being activated when the underlaid incoming audio signal is abovea specific amplitude, and with the audio signal being passed onunchanged below the specific amplitude. This allows computationcomplexity to be saved since overdriving (or underdriving) can beidentified in good time, so that the dynamic compression may beactivated only when it is required.

The invention will be explained in the following text using exemplaryembodiments which are illustrated in the figures of the drawing, inwhich:

FIG. 1 shows an exemplary embodiment of a circuit arrangement accordingto the invention, and

FIG. 2 shows a vector diagram in the spectral plane before and afterlimiting by means of a circuit arrangement as shown in FIG. 1.

In the exemplary embodiment of a circuit arrangement 1, as shown in FIG.1, for reducing the dynamic range of an incoming audio signal 2, atransformation device 3 is provided, which converts the incoming audiosignal 2 to a complex input spectrum 4 by means of fast Fouriertransformation (FFT).

The complex input spectrum 4 is then supplied to a spectral processingdevice 5, which is connected downstream from the transformation deviceand changes the complex input spectrum 4 so as to produce a complexoutput spectrum 6 whose resultant dynamic range is restricted incomparison to that of the input spectrum 4. The signal processing in thespectral processing device 5 in this case provides for the individualspectral lines to be evaluated, and to be compared with a limit value 7.A processing unit 8 is provided for this purpose, which detects spectrallines whose amplitude is greater than the limit value 7 and attenuatesthem to such an extent that they are below the limit value 7.

A further processing unit 9 determines areas in the input spectrum 4whose amplitude and/or respective spectral range mean/means that theyare not psychoacoustically audible. These include, for example, thespectral and time masking of noise. Spectral masking in this case meansthat a low-frequency tone can completely mask a somewhat higher tone ata lower level, so that the higher tone can no longer be perceived by thelistener. A tone which cannot be perceived in this way is identified bythe processing unit 9, and is completely removed from the overallsignal. In the case of time masking, an acoustic signal at a low levelfollows a signal at a high level, with the signal which occurs first andis at the high level making the subsequent signal, which is at a lowlevel, inaudible for a certain time. The processing unit 9 alsoidentifies and eliminates the respective spectral range on the basis ofthis behaviour. The relationships relating to psychoacoustical maskingare described, by way of example, in B. Zwicker, “Psychoakustik”[Psychoacoustics], Springer-Verlag 1982, pages 35 to 46 and 93 to 101.The processing unit 9 controls the relationships mentioned therein.

A further processing unit 10 takes account of the spectral position ofthe individual spectral lines, with very high and very low tones beingattenuated to a greater extent than tones in the central range. Thisresults in a loudness function so that high and low frequencies areemphasized when the drive level is low (corresponding to low levels),thus corresponding to the method in which the human hearing operates,which perceives high and low tones at high levels better than at lowlevels.

If the processing units 8 to 10 are used to provide a limiter function,two further processing units 11 and 12 are provided, which can alsogenerally be used for dynamic compression. The processing unit 11 inthis case detects one or more maxima in the input spectrum 4 andattenuates them by a specific amount, with the overall gain, that is tosay the amplitudes of all the signals, being emphasized at the same timein the present case. The amplitude peaks are thus reduced, whilerelatively low levels are correspondingly emphasized at the same time.

In contrast, the processing unit 12 evaluates relatively smallamplitudes, which are greater than a third limit value 13 whichcorresponds to a specific maximum background noise level, but are belowa fourth limit value 14, which is higher than the third limit value 13.The two processing units 11 and 12 follow one another in this case, thusresulting, overall, in the dynamic range being tailored “from the topand bottom”, and then being further emphasized above the backgroundnoise level.

The processing units 8, 9, 10 and 12 are followed by a selection device15 which selects the processing form (which is predetermined for exampleas a function of the amplitude distribution and spectral composition ofthe input spectrum 4 determined by the respective processing unit 8 to12), or combines a number of processing forms with one another. Theselection device 15 then uses this to produce the output spectrum 6which, in the present case, is likewise complex, but may also be purelyreal in certain applications.

The output spectrum 6 is supplied to a further transformation devicewhich uses inverse fast Fourier transformation (IFFT) to produce a timesignal once again, namely an output audio signal 17.

In the present exemplary embodiment, the transformation device 3 ispreceded by an advanced initiation device 18, which supplies theincoming audio signal 2′, delayed by means of a delay device 19, as theincoming audio signal 2 to the transformation device 3. In this case, acontrol device 20 activates or deactivates the circuit arrangement 1within the delay time, depending on whether the incoming audio signal 2′is or is not greater than a specific limit value. If the specificamplitude limit value is exceeded, then the circuit arrangement 1 isactivated during the time in which the signal is passing through thedelay device 19, so that the circuit arrangement 1 is ready to operatewhen the increased amplitude value occurs at the output of the delaydevice 19.

In this case, the output audio signal 17 is passed to the output 22 to aswitching device 21 which is connected downstream from thetransformation device 16. Otherwise, the incoming audio signal 2′ ispassed to the output 22.

FIG. 2 shows the situation before (FIG. 2 a) and after (FIG. 2 b)limiting for four spectral lines Z1 to Z4 in two vector diagrams (FIGS.2 a, 2 b), with the phase angle in particular being taken into accountin this case. In the case before limiting, it is assumed that all fourspectral lines have the sample amplitude, with the spectral lines Z1 andZ4 having opposite phases. This results in an overall vector ZT which isgreater than a maximum permissible amplitude value TA. Limiting is nowintended to be carried out such that the overall phase does not change.The spectral lines Z2 and Z4 are in this case not used for a reductionprocess, since they do not contribute to the overall vector. Inconsequence, the magnitudes of the vectors for the spectral lines Z1 andZ3 are reduced to such an extent that the overall vector ZT is no longergreater than the amplitude threshold value AT.

1. Circuit arrangement for reducing the dynamic range of audio signals,comprising: a first transformation device to which an incoming audiosignal is supplied and which transforms this audio signal from the timedomain to the frequency domain resulting in an input spectrum, aspectral processing device, which is connected downstream from the firsttransformation devices, to receive the input spectrum and use it toproduce an output spectrum such that the output spectrum has a narrowerdynamic range than the input spectrum, and a transformation device,which is connected downstream from the spectral processing device, issupplied with the output spectrum and transforms this output spectrumfrom the frequency domain to the time domain, resulting in an outputaudio signal.
 2. Circuit arrangement according to claim 1, in which thefirst transformation device operates using the fast Fouriertransformation, and the second transformation device operates using theinverse fast Fourier transformation.
 3. Circuit arrangement according toclaim 1, in which the spectral processing device attenuates theamplitude in those areas of the spectrum in which the amplitude and/orthe spectral range satisfy/satisfies one or more conditions.
 4. Circuitarrangement according to claim 3, in which one condition is that theamplitude is greater than a first limit value in one spectral range. 5.Circuit arrangement according to claim 3, in which one condition is thata signal at the respective amplitude and in the respective spectralrange is not psychoacoustically audible.
 6. Circuit arrangementaccording to claim 3, in which one condition is that the respectiveamplitude is a maximum or one of the maxima.
 7. Circuit arrangementaccording to claim 3, in which one condition is given by the position ofthe respective spectral range in the overall spectrum.
 8. Circuitarrangement according to claim 1, in which relatively small amplitudes,which are greater than a second limit value which corresponds to aspecific basic noise level and are less than a third limit value whichis higher than the second limit value are emphasized.
 9. The circuitarrangement as claimed in claim 1, in which the input spectrum iscomplex.
 10. Circuit arrangement according to claim 1, in which thefirst transformation device is preceded by an advanced initiationdevice, which supplies the incoming audio signal with a delay to thefirst transformation device, with both transformation devices as well asthe processing device being activated when the underlaid incoming audiosignal is above a specific amplitude, and with the audio signal beingpassed on unchanged below the specific amplitude.